-LRB- CNN -RRB- -- A fire erupted Friday at the headquarters of the Shanksville , Pennsylvania , memorial to United Airlines Flight 93 , which crashed on September 11 , 2001 , officials said .

`` There is a potential for 9/11 memorabilia loss due to a fire , '' according to a statement from the National Park Service , but the extent of the damage is not known yet .

NPS spokesman Mike Litterst said 10 % of the archives and museum collection was stored in the damaged buildings , but much of it was kept in a fireproof safe .

Among the items in the damaged buildings was a U.S. flag that flew over the U.S. Capitol on the day of the terrorist attack . The flag was given to the Flight 93 National Memorial last September 11 . Its status was not known .

Four buildings were damaged , the park service said . Heavy rain helped the firefighting efforts .

Seven fire companies responded to the blaze , which started about 3 p.m. , said Geraldine Budzina , a Somerset County dispatcher . No injuries were reported .

`` I think stuff has been lost ''

Ken Nacke , whose brother Louis was on Flight 93 , said many 9/11 artifacts -- including notes and other mementos left at a temporary memorial at the crash site -- were being stored in the area where the fire burned .

He said he had spoken Friday with the head of Families of Flight 93 shortly after the group president was contacted by the memorial 's superintendent .

`` I think stuff has been lost , '' Nacke said . `` I just do n't know what has been lost . I know the layout of the buildings and I would n't be surprised . It 's heartbreaking that we spent all this time to have this happen . All the blood sweat and tears that went into building this . ''

Nacke helped raise funds for the Flight 93 National Memorial and advised in its planning .

`` At the temporary memorial , people would leave cards , T-shirts and handmade stuff that was very comforting to us , '' he said . `` That 's why it 's heartbreaking . I hope none of this stuff is lost . ''

Cause of fire is unknown

Clouds of dark smoke could be seen billowing into the blue sky over the grounds Friday afternoon on a live webcam .

`` We are deeply saddened to learn that a fire occurred at the Flight 93 National Memorial headquarters , '' Gordon Felt , president of Families of Flight 93 said in a statement . `` We understand that no one was injured , and we are grateful for that . ''

`` Neither the memorial proper nor the new visitor center currently under construction were affected , as the headquarters is located approximately two miles from those sites , '' Litterst said .

Initial reports were of extensive damage to the complex , Litterst said . All employees and volunteers were safely evacuated .

The cause of the blaze is under investigation , he said .

40 people died in the plane crash

The memorial park is dedicated to the 40 passengers and crew who died when Flight 93 crashed outside the town in southwestern Pennsylvania . The plane went down , killing all on board , as passengers fought back against the hijackers , according to investigations .

The memorial , still incomplete , includes a visitor center with traditional and interactive exhibits , public programs and information about the history of Flight 93 .

United Airlines Flight 93 was traveling from Newark , New Jersey , to San Francisco when hijackers took over the plane , according to the 9/11 Commission .

Investigators said the terrorists were most likely trying to turn the airplane toward Washington to hit a major political target .

The 2,200-acre memorial park is managed by the National Park Service . The second phase of construction is to be completed by 2015 .

Since 2001 , more than 1 million people from around the world have visited the crash site .

What you need to know about the Flight 93 National Memorial

CNN 's Aaron Cooper , Deanna Hackney and Laura Bernardini contributed to this report .

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NEW : Some 911 artifacts may have been damaged , park service says

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Fire breaks out at headquarters of memorial to United Airlines Flight 93

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Headquarters complex is about two miles from the memorial in Pa. .

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Flight 93 crashed on September 11 , 2001 , after hijackers took over the plane